1. Field of the Invention
The present invention relates to a direct-injection spark-ignition engine having an injector for injecting fuel directly into a combustion chamber. More particularly, the invention pertains to the construction of a direct-injection spark-ignition engine in which a stratified combustible mixture is produced around a spark plug using a tumble during operation by stratified charge combustion.
2. Description of the Related Art
In one example of a conventionally known direct-injection spark-ignition engine which is provided with an injector for injecting fuel directly into a combustion chamber, the air-fuel ratio is increased to produce a lean mixture and the injector injects the fuel during every compression stroke such that the mixture is locally distributed around a spark plug to cause stratified charge combustion in a low-speed low-load operating range, thereby achieving an improvement in fuel economy. For this kind of engines, various approaches to accelerating mixture strification have been proposed heretofore involving a variety of piston head shapes.
As an example, there is formed a deep recess, or a cavity, in the top surface of a piston at a location offset from the axis of a cylinder. An injector injects the fuel toward this cavity, producing a controlled mist of fuel which moves toward an area surrounding a spark plug after hitting against the surface of the cavity.
Another example of a conventional direct-injection spark-ignition engine is disclosed in Japanese Unexamined Patent Publication No. 2000-120440, in which an intake line is so constructed as to produce a tumble in a combustion chamber and a cavity having an xcfx89-shaped cross section is formed in the top surface of a piston. The xcfx89-shaped cavity is formed of an arc-shaped recess cut in an area of the piston head close to an intake valve and another arc-shaped recess cut in an area of the piston head close to an exhaust valve, the two arc-shaped recesses being located adjacent to each other, forming a central ridge in between. In this engine, an injector provided in a peripheral wall of the combustion chamber injects fuel toward the arc-shaped recess close to the intake valve and the tumble flows along the arc-shaped recess formed close to the exhaust valve. Guided along surfaces of the respective arc-shaped recesses, a fuel spray and the tumble meet at the central ridge on the piston head and are directed upward together, whereby the fuel spray joined by the tumble is brought toward a spark plug provided at a central upper part of the combustion chamber.
In the aforementioned arrangement in which the fuel is injected in the compression stroke, the surface of the cavity is situated close to the spark plug at each fuel injection point (which exists in a latter part of the compression stroke) during stratified charge combustion operation. Even after ignition, the surface of the cavity is close to the spark plug during an early stage of combustion wherein the piston is still in the proximity of its top dead center. Thus, flame propagation tends to be hindered by the cavity surface during this stage. Another problem of this approach is that the amount of hydrocarbon (HC) emissions could increase due to fuel adhesion to the cavity surface.
In the aforementioned approach of Japanese Unexamined Patent Publication No. 2000-120440 employing the piston having the co-shaped cavity in its top surface, the fuel spray and the tumble flowing along the surfaces of the arc-shaped recesses formed close to the intake and exhaust valves, respectively, are guided upward together toward the central upper part of the combustion chamber. Although this construction provides an enhanced efficiency in terms of mixture transport toward the spark plug, the mixture is carried relatively swiftly by the tumble so that the mixture passes the nearby area of the spark plug within quite a short period. For this reason, the time period during which the combustible mixture exists around the spark plug (and the mixture can be ignited by the spark plug) is extremely short and, therefore, there is provided a small degree of freedom in determining fuel injection and ignition timing. Furthermore, flame propagation tends to be hindered by the central ridge in the xcfx89-shaped cavity during an early stage of combustion. This construction also has a problem that the injected fuel tends to adhere to the surface of the arc-shaped recess close to the intake valve.
Under these circumstances, the inventor of the present invention has introduced a new engine design for properly controlling the flow of air-fuel mixture. According to this design, an injector sprays a mist of fuel into a combustion chamber from a direction approximately opposite to the flow of tumble. As the fuel spray injected from the injector collides with the tumble, evaporation and atomization of the fuel are accelerated and the fuel spray decelerated by the tumble is mixed with air. As a result, a combustible mixture stays longer around a spark plug in the form of suspended matter.
An example of piston structure (comparative example) according to this new design is shown in FIG. 12.
Referring to FIG. 12, a spark plug 111 is provided at a central upper part of a combustion chamber 105 and an injector 112 is provided at its peripheral part. There is formed a cavity 115 in the top surface of a piston 104 so that a fuel spray and a tumble T collide with each other from approximately opposite directions within this cavity 111. More specifically, as illustrated in the cross section of FIG. 12, there are provided an intake port 107 on the left side and an exhaust port 108 on the right side in a ceiling of the combustion chamber 105, and the tumble T created in the combustion chamber 105 flows clockwise. The spark plug 111 is located at the central upper part of the combustion chamber 105 and the injector 112 at the peripheral part of the combustion chamber 105 on the left side (on the side of the intake port 107) as illustrated. The bottom of the cavity 115 formed in the top surface of the piston 104 has a smooth concave shape whose lowest point exists on or near cylinder axis Z. During the stratified charge combustion operation, the tumble T (particularly a later-described counterflow of tumble T) and the fuel spray collide with each other from approximately the opposite directions within the cavity 115 so that a combustible mixture stays around the spark plug 111 for a prolonged period.
In this structure, it is necessary that the fuel spray and the tumble T collide with almost an equal strength in approximately a central part of the cavity 115 to cause the combustible mixture to stay around the spark plug 111 for a prolonged period. It is also necessary to retard the fuel injection point (so that it approaches the ignition point) as much as possible for avoiding dispersion of fuel and to increase fuel pressure so that a required amount of fuel can be injected within a short period of time. Accordingly, it is desirable to produce a strong tumble within the cavity 115 at the point of fuel injection so that the tumble would not be much disturbed by the increased fuel pressure. With the aforementioned structure of the cavity 115 having approximately a left-right symmetric profile with respect to the cylinder axis Z (FIG. 12), however, the center of the tumble could be displaced from the cylinder axis Z and the tumble itself is likely to diminish during middle to latter parts of the compression stroke as will be explained later with reference to FIGS. 13A-13C and 14. Thus, there is still left some room for improvement in the structure of FIG. 12 concerning, in particular, creation of a strong tumble at the point of fuel injection.
In view of the foregoing, it is an object of the invention to provide a direct-injection spark-ignition engine which causes a mist of fuel sprayed from an injector and a tumble to collide with each other from approximately opposite directions so that a combustible mixture stays around a spark plug for a prolonged period of time during stratified charge combustion operation. It is a more specific object of the invention to provide a direct-injection spark-ignition engine capable of maintaining a sufficiently strong tumble up to every fuel injection point so that fuel is evaporated and atomized and a stratified mixture is created in a preferable fashion during stratified charge combustion operation.
According to the invention, a direct-injection spark-ignition engine having an intake air system which is so constructed as to produce a tumble in a combustion chamber whose ceiling is higher at a central part than at a peripheral part as seen in a cross section viewed from a direction from which the tumble seems to be turning clockwise comprises a spark plug and an injector provided at the central part and the peripheral part of the ceiling, respectively. In this direct-injection spark-ignition engine, the injector injects fuel into the combustion chamber in a direction opposite to a flow of the tumble to produce a stratified combustible mixture around the spark plug at an ignition point during stratified charge combustion operation, and a cavity whose upper opening is elongated to both the left and right sides of a cylinder axis is formed in the top surface of a piston, the distance between the ceiling of the combustion chamber and a bottom surface of the cavity as measured parallel to the cylinder axis being smaller on the right side of the cylinder axis than on the left side thereof and largest at least at a point where the cylinder axis crosses the bottom surface of the cavity, and a portion of the cavity to the left of the cylinder axis has a larger volumetric capacity than a portion of the cavity to the right of the cylinder axis, as seen in the aforesaid cross section.
In this construction, the injector injects the fuel during a compression stroke in such a manner that a fuel spray and the tumble collide with each other in the cavity from approximately opposite directions during the stratified charge combustion operation. This serves to accelerate atomization of the fuel and produce the stratified combustible mixture around the spark plug located at the central part of the ceiling of the combustion chamber by stratifying a mixture of the fuel spray and air. The fuel spray is decelerated by its collision with the tumble, creating a situation in which the mixture floats beneath the central part of the ceiling of the combustion chamber. As a result, compared to the aforementioned prior art arrangement of Japanese Unexamined Patent Publication No. 2000-120440 in which the mixture is simply transported by the tumble, the period of time during which the combustible mixture stays around the spark plug is extended, resulting in an increase in the degree of freedom in determining fuel injection and ignition timing. The arrangement of this invention also serves to reduce adhesion of the fuel to the cavity surface and increase the effect of improving fuel economy by mixture strification.
These and other objects, features, and advantages of the present invention will become more apparent upon reading the following detailed description along with the accompanying drawings.